DD patent 110 507 discloses a process for preparing copolyamides from caprolactam and salts of diamines and dicarboxylic acids, e.g. AH salt, in which caprolactam, AH salt and water are supplied to a VK tube (vertical condensation tube) from the top, and the corresponding copolyamide is withdrawn at the lower section. The process has the disadvantage that diamine is also discharged with the vapors which escape at the top of the VK tube and is lost. The process also has the disadvantage that the water released with the AH salt solution supplied considerably impairs a precise temperature regime at the top of the VK tube.
Avoiding these disadvantages, EP-A-0 393 546 describes a continuous process for preparing copolyamides from caprolactam and salts of diamines and dicarboxylic acids, in which caprolactam together with polyamide-forming compounds formed from dicarboxylic acids and diamines is passed from the top downward through a vertical polymerization tube at polyamide-forming temperatures under elevated pressure and with simultaneous evaporation of water by means of a tubular preliminary condensation zone. This forms a vapor phase and a prepolymer at a temperature above the melting point of the prepolymer. The vapor phase is separated from the prepolymer melt and conducted into a column in which water vapor and an aqueous diamine solution are separated and the aqueous solution comprising diamines is recycled into the polymerization. Subsequently, the prepolymer melt is mixed with molten caprolactam, and the mixture of prepolymer and caprolactam is passed from the top downward through a vertical polymerization tube, in order thus to obtain a copolyamide.
The disadvantage of this process is that the prepolymer from the preliminary condensation zone already consists of blocks of low molecular weight condensed diamines and dicarboxylic acids, which also still remain as blocks in the subsequent polymerization with caprolactam. A better random distribution of the components does not take place. More salt formed from diamines and dicarboxylic acids is thus required in order to arrive at the desired copolymer melting point. In addition, copolymers thus prepared have a greater tendency to become discolored.